Human CNS-Apolipoprotein E Isoform Production and Clearance A. Specific Aims: Alzheimer's disease (AD) is a common, devastating disease. 5 million Americans are suffering from AD, and it is currently estimated that by 2050 this number will triple. The only major and wellvalidated genetic risk factor for AD is one's Apolipoprotein E (ApoE) genotype. One ApoE4 allele increases the risk of developing AD by 3-fold, while two copies of ApoE4 increase this risk by 12-fold. A reduced risk for AD is associated with the ApoE2 allele. The metabolism and function of ApoE is partially understood in the periphery, however very little is known about ApoE in the central nervous system (CNS). Because ApoE plays such an important role in the CNS, and because its metabolism in the CNS is not well understood, we propose to determine the physiology and possible isoform pathophysiology as it relates to AD. Isoform dependent changes in ApoE metabolism and amount (and subsequently transport of AP) may underiie the increased risk of AD in humans. Hypothesis 1: ApoE4 amount is less in the brain and CSF vs. ApoE3. Hypothesis 2: ApoE4 clearance rate or turn-over rate if faster in the CNS compared to ApoE3. Specific Aim: Utilizing a recently developed mass spectrometry assay to measure ApoE isoforms independently, we will measure ApoE4 and ApoE3 in the human brain and CSF. In addition, the in vivo metabolism of human ApoE will be measured using stable isotope-labeling and mass spectrometry in 60 cognitively normal or dementia of the Alzheimer's type (DAT) participants with E2, E3 or E4 ApoE alleles. These participants will be recruited from the Washington University ADRC. B. Relevance to Alzheimer's Disease: ApoE is the strongest genetic risk factor for Alzheimer's disease, and is a potential target for disease-modifying therapies. Our data will provide an important link regarding how ApoE4 may be involved in the pathophysiology of Alzheimer's disease, and may lead to improved therapeutics, which target the major genetic risk factor of Alzheimer's disease, ApoE4.